In case of the place which has airtight space like as a refrigerator or a freezing warehouse, the internal pressure of airtight space is different from the external pressure and there are many cases that the door of airtight space may not be easily opened.
In order to solve this problem, the devices have been invented which may adjust the internal pressure in order to equalize the internal pressure of airtight space to the external pressure.
However, the prior internal pressure equilibrium devices have a problem that pressure equilibrium operation may not be performed rapidly and the structure is complex so that the manufacture cost may be increased.